Three phase AC motors are generally simpler, more reliable and more efficient than single phase AC motors. In addition to three phase AC motors, much high-power industrial equipment requires three phase AC power. Three phase AC power is generally supplied to industrial areas. However, only single phase AC power is available to most residential and rural areas.
The single phase AC power available in most residential and rural areas is provided by a step down transformer connected a high voltage line and, in the United States, is normally supplied as about 240 volts at 60 Hz between the first and second input lines. The transformer is generally center tapped with a neutral line to provide two phases of about 120 volts that are separated by 180 degrees.
Phase converters and inverters convert single phase AC power to three phase AC power to power three phase motors. Phase converters generate a second voltage that is out of phase with the input voltage. The first phase is the voltage between the first and second input line, the second phase is the voltage between the first input line and the second voltage and the third phase is the voltage between the second input line and the second voltage. Three equal phases spaced 120 degrees apart are provided if the second voltage has an amplitude of 3/2 times the amplitude of the input voltage and is 90 degrees out of phase with the input voltage.
The two types of phase converters generally available are the static phase converter and the rotary phase converter. In prior known static phase converters for use with inductive loads two terminals from the input supply were connected to two of the windings of a three phase motor and a capacitor was connected in series between the third winding and one of the terminals from the input supply. The capacitor in combination with the inductive load creates a lead circuit to provide the out of phase second voltage.
Such phase converters are disclosed in U.S. Pat. No. 4,492,911 to Molitor, U.S. Pat. No. 4,777,421 to West, U.S. Pat. No. 3,673,480 to Johnstone and U.S. Pat. No. 5,621,296 to Werner et al. This type of phase converter includes a large capacitor for starting the motor and a smaller capacitor for running the motor. This type of phase converter is relatively inexpensive, however this type of phase converter can only be used with inductive loads. The capacitor must be selected for the specific inductive load to provide the correct phase shift. Also, the amplitude of the voltage out of the capacitor is at most one half the input voltage so this type of phase converter cannot provide balanced currents to the windings at varying loads. Unbalanced currents cause localized heating so that three phase motors run with this type of static phase converter can only be run at a fraction of the rated capacity.
U.S. Pat. No. 5,293,108 to Spudich discloses a static phase converter that includes a balancing coil between the two input lines and a capacitor connected between one input line and the third winding to shift the phase. As in the previously described static phase converters, two terminals from the input supply were connected to the first and second windings of a three phase motor, and a start capacitor and a smaller run capacitor are provided. The balancing coil and capacitor must be selected to match the impedance of the three phase load with this converter.
Rotary phase converters use motor-generators powered by single phase AC power to generate the second voltage signal. Rotary phase converters are disclosed in U.S. Pat. No. 4,656,575 to West, U.S. Pat. No. 5,065,305 to Rich, and U.S. Pat. No. 5,187,654 to Felippe. Rotary phase converters are generally more complex, more expensive and less efficient than static phase converters, and produce an unbalanced output which causes severe imbalances in the phase currents of three phase motors.
Inverters convert the entire single phase AC input voltage to a DC voltage with rectifiers and convert the DC voltage into three balanced AC phases with an inverter circuit. Examples of inverters are disclosed in U.S. Pat. No. 4,855,652 to Yamashita et al., U.S. Pat. No. 5,793,623 to Kawashima et al., U.S. Pat. No. 4,849,950 to Sugiura et al. and U.S. Pat. No. 4,978,894 to Takahara. The inverter circuit requires a minimum of six transistors and six diodes as well as control electronics for all of the transistor. Inverters are generally more complex and more expensive than static phase converters. Since the entire single phase AC input voltage is converted to DC, inverters are inherently less efficient than static phase converters. The output voltage of inverters consists of a pulse-width-modulated (PWM) signal with a high harmonic content, limiting their application to inductive loads. The high frequency harmonics present in the output voltages cause unwanted reflections in the wires connecting the inverter to the motor load, and limit the acceptable distance between the inverter and the motor.